This issue happens to me usually after I get the error code 1. It corrects itself over time, but what I think helps is to quit and reopen the world, or remove Minecraft from your history and reopen Minecraft.
This only happens when the wither is spawned using a skeleton egg. Naturally spawning withers use their sword.
Actually, I found another wither and this appears to be a bug that happens sometimes. Sometimes they shoot arrows other times they use their swords.
Really? I read that you would be able to do this in 0.12.1. Link is here: http://minecraftpocketedition.wikia.com/wiki/Update_0.12.1
Go down to tweaks, blocks/items.
Thanks man I really needed a way to remove that issue. Now I can play normally again!